Catalytic converters are devices used to reduce pollutants from motor vehicle exhaust gases. Catalysts promote chemical reactions that convert pollutants such as carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NOx) into carbon dioxide, water, and nitrogen. The United States government stringently regulates the emission levels of these pollutants.
Oxygen sensors are used to monitor the performance of a catalyst by detecting the oxygen levels in the exhaust gas both upstream and downstream from the converter. Generally, when a catalyst deteriorates it loses part of its oxygen storage capacity and thus, part of the untreated exhaust gas breaks through the catalyst causing the downstream oxygen sensor to deviate from a neutral, stoichiometric position.
Using a present method, an aged catalyst may be detected by first biasing the oxygen level in the exhaust stream to a rich or lean amount. The present method then maintains counts of rich/lean and lean/rich transitions of the upstream and downstream oxygen sensors. A ratio of the counts is then used to determine the amount of untreated exhaust gas breaking through the catalyst. While this method is generally effective, it also suffers drawbacks making it only unsatisfactorily suitable for future vehicles.
First, the existing detection method is intrusive to the engine fueling control system and thereby increases pollutant emissions while the method executes. This intrusive property makes it increasingly difficult to simultaneously satisfy the requirements of detecting an aged catalyst and maintaining low levels of pollutant emissions.
Second, since pollutant emissions are increased while the existing method executes there exists an urgency to conclude the method as quickly as possible. This urgency in execution may lead to incorrect conclusions as to whether the catalyst is aged. Such incorrect conclusions may result in unnecessary expense for catalyst replacement.
Third, the existing method has difficulty differentiating between rich/lean oxygen sensor switching that is caused by exhaust gas and similar switching that is caused by electrical noise or a hypersensitive oxygen sensing element. While software filtering may be used to help differentiate between the causes of rich/lean switching, such software filters are often complex.